Circular knitting machines are known include a first needle supporting element- e.g. a cylinder, having a plurality of radial grooves defined along its periphery in which the needles are accommodated and can slide axially. Each needle includes a heel or butt having a predetermined height protruding from the grooves for engaging within a path defined by a plurality of shaped cams which are mounted on a supporting structure and are arranged with a front cam surface facing the needle supporting element.
In circular machines having two needle beds, two needle supporting elements are perpendicularly arranged, since a dial is arranged above the cylinder having a plurality of radial grooves in which other similar needles are accommodated and face the needles carried by the cylinder. Similarly, the needles of the dial also have at least one protruding heel which protrudes upwardly from the grooves of the dial and engages within paths defined by other cams mounted on a supporting structure arranged upwardly facing the dial.
The relative motion between the supporting structures of the cams and the respective needle supporting element causes the needles to follow the paths defined by the cams, moving the needles along the respective grooves of the cylinder and dial to grip the thread fed by thread guides and to form stitches. In circular machines, the relative motion occurs about the axis of the knitting machine, which coincides with the axis of the cylinder and with the axis of the dial.
Known stitch cams, such as those described in U.S. Pat. No. 5,182,927 to Pernick, lower the needles of the cylinder or move the needles of the dial after the needles have engaged the thread fed by a thread guide, to cast off the previously formed stitches in a synchronized manner. The stitch cam according to the above patent includes a fixed lower cam portion and an upper movable cam portion which are each mounted to a supporting body. The lower movable portion is adjustable from a minimum to a maximum along a line which is parallel to the angular slope of the cam to move diagonally. This adjustment is made externally through the use of an cylindrical cam utilizing an Archimedean spiral which is contained within the supporting body and is interconnected to the movable portion which slides along an obliquely angled surface of the supporting body to raise and lower the needles by a corresponding amount.
A number of problems are encountered in the above described stitch cam. First, the cam requires sliding contact between the obliquely angled surface of the upper movable portion and the corresponding angled surface of the supporting body. Therefore, dissimilar materials cannot be used for the contacting parts without producing excessive wear and producing frictional forces which could render the cam partially or completely inoperative.
A second problem is the requirement of an angled contacting surface in the supporting body which requires additional time and labor to fabricate and introduces additional potential tolerancing dilemmas between the three major components, ie: the supporting body, the movable upper cam portion and the lower fixed cam portions. Alignment between the three components must be tightly held in order to provide smoothness of movement, which is critical, particularly given the high speed and synchronous steps involved in an automatic circular knitting production operation. Variations in tolerancing may also occur when the movable cam portion is shifted to change the stitch level, wherein contacting portions of the needle separate from the heel may contact the surfaces of the fixed cam portion and the movable cam portion, thereby increasing the probability of needle breakage against a discontinuous portion or the cylinder in the case of adjacent cam assemblies having different stitch levels, and ultimately causing unwanted production delays.
Yet another problem is an inability for the needles to transition between adjacently disposed cam units on a cam supporting bed in the circular knitting machine, introducing a greater probability of needle breakage due to excessive vibration due caused by discontinuity in between adjacent closed cam tracks, particularly in machines having a smaller radius, and therefore being more pronounced between cams with different stitch levels.
Still another problem generally encountered in stitch cam units of the prior art is an inability to provide radial adjustment; that is, relative to the cylinder or other needle supporting element facing the cam.